The invention relates to an arrangement for preventing false alarms of a passive infrared motion alarm.
An arrangement of this kind is known and described, for example, in German Offenlegungsschrift No. 27 34 157 corresponding to U.S. application Ser. No. 924,163, incorporated herein by reference. The fundamental principle is a passive infrared motion alarm which is known, for example, from German Auslegeschrift No. 21 03 909 corresponding to U.S. Pat. No. 3,703,718, incorporated herein by reference. In an infrared motion alarm of this type, changes in incoming thermal radiation are converted into electric signals. Radiation in the wavelength range of greater than approximately 4.5 .mu.m is utilized. In German Offenlegungschrift No. 27 34 157 the fact is taken into account that electromagnetic radiation having a wavelength less than approximately 4.5 .mu.m, thus at wavelengths smaller than the infrared useful range, is withheld from the infrared detector by an appropriate optical filter, but that the radiation withheld by this filter, i.e. absorbed, heats the filter itself and can trigger false alarms in the infrared detector together with the analysis device connected thereto, via the characteristic thermal radiation of the filter which is thus produced. Above all this can be the case if strong light sources, such as car headlights irradiate from the outside of the room which is monitored by the motion alarm through window panes into the monitored room. On the one hand window glass has the advantage that electromagnetic radiation having a wavelength of greater than approximately 4.3 .mu.m is absorbed and that infrared radiation from outside the monitored room does therefore not interfere with the monitoring, but on the other hand allows electromagnetic radiation to pass below this range. This radiation can then trigger false alarms via the secondary effect of the filter heating. In particular, this is the case when the interfering light source has characteristic changes in radiation as is the case in moving car headlights. Their movements lie in the range to which the analysis device responds.
In order to solve this problem it is known from German Offenlegungschrift No. 27 34 157 to provide existing focusing devices with additional filtering actions and to withhold undesired radiation from the actual optical filter either by means of selective absorption or selective reflection. However, an adequately strong filter action can either not be achieved or only with great technical expense, primarily because of the large surfaces of the focusing elements.